A conventional wide code division multiple access (WCDMA)-based wireless communication method is a very effective wireless transmission method in which voice-based data is transmitted at a low speed and a soft handover is taken into consideration, but is ineffective when data is transmitted at a high speed in a multi-path fading environment. An evolved-universal mobile telecommunications system (E-UMTS) proposes a downlink transmission speed of about 100 Mbps. In the E-UMTS, as a multiple access technique, orthogonal frequency division multiplexing (OFDM) is mainly concerned in downlink and a discrete Fourier transform spread OFDM (DFT-S-OFDM) is mainly concerned in uplink in order to minimize a peak-to-average-power-ratio (PAPR) of a user equipment (UE).
FIG. 1 shows a structure of a wireless communication system. The wireless communication system may be have a network structure of an E-UMTS. The E-UMTS may be referred to as a long-term evolution (LTE) system. The wireless communication system can be widely deployed to provide a variety of communication services, such as voices, packet data, etc.
Referring to FIG. 1, a E-UMTS is classified into an evolved-UMTS terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC). The E-UTRAN includes at least one base station (BS) 20. A user equipment (UE) 10 may be fixed or mobile, and may be referred to as another terminology such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, etc. The BS 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as an evolved node-B (eNB), a base transceiver system (BTS), an access point, etc. There are one or more cells within the coverage of the BS 20.
Interfaces for transmitting user traffic or control traffic may be used between the BSs 20. Hereinafter, downlink is defined as communication from the BS 20 to the UE 10, and uplink is defined is communication from the UE 10 to the BS 20.
The BS 20 provides the UE 10 with an end-to-end point of a user plane and a control plane. The BSs 20 are interconnected by means of an X2 interface, and may have a meshed network structure in which the X2 interface always exists between the neighboring BSs 20.
The BSs 20 are also connected by means of an S1 interface to the EPC, more specifically, to an access gateway (aGW) 30. The aGW 30 provides an end-to-end point for a session and mobility management function of the UE 10. The S1 interface may be provided between the BS 20 and the aGW 30 so that a plurality of nodes can be interconnected in a many-to-many manner. The aGW 30 can be classified into a part for processing user traffic and a part for processing control traffic. In this case, for inter-communication, a new interface may be used between an aGW for processing new user traffic and an aGW for processing new control traffic. The aGW 30 is also referred to as a mobility management entity user plane entity (MME/UPE).
Layers of a radio interface protocol between a UE and a network can be classified into L1 layer (a first layer), L2 layer (a second layer), and L3 layer (a third layer) based on the lower three layers of the open system interconnection (OSI) model that is well-known in a communication system. A physical layer belongs to the first layer and provides an information transfer service on a physical channel. A radio resource control (RRC) layer belongs to the third layer and serves to control radio resources between the UE and the network. The UE and the network exchange RRC messages via the RRC layer. The RRC layer may be located in network nodes (i.e., a BS, an aGW, etc.) in a distributed manner, or may be located only in the BS or the aGW.
The radio interface protocol horizontally includes a physical layer, a data link layer, and a network layer, and vertically includes a user plane for data information transfer and a control plane for control signaling delivery.
FIG. 2 is a diagram showing a control plane of a radio interface protocol. FIG. 3 is a diagram showing a user plane of the radio interface protocol. In FIGS. 2 and 3, a structure of the radio interface protocol between a UE and an E-UTRAN is based on the third generation partnership project (3GPP) wireless access network standard.
Referring to FIGS. 2 and 3, a physical layer belonging to a first layer provides an upper layer with an information transfer service on a physical channel. The physical layer is coupled with at media access control (MAC) layer, i.e., an upper layer of the physical layer, via a transport channel. Data is transferred between the MAC layer and the physical layer on the transport channel. In addition, data is transferred between different physical layers, i.e., between physical layers of a transmitting side and a receiving side.
The MAC layer in a second layer provides services to a radio link control (RLC) layer, i.e., an upper layer of the MAC layer, via a logical channel. The RLC layer in the second layer supports reliable data transfer. Functions of the RLC layer can be implemented as a function block included in the MAC layer. In this case, as indicated by a dotted line, the RLC layer may not exist.
A packet data convergence protocol (PDCP) belonging to the second layer performs a header compression function. When transmitting an Internet protocol (IP) packet such as an IPv4 packet or an IPv6 packet, the header of the IP packet may contain relatively large and unnecessary control information. The PDCP layer reduces the header size of the IP packet so as to efficiently transmit the IP packet through a radio interface.
An RRC layer belonging to a third layer is defined only in the control plane. The RRC layer serves to control the logical channel, the transport channel, and the physical channel in association with configuration, reconfiguration, and release of radio bearers (RBs). An RB is a service provided by the second layer for data transmission between the UE and the E-UTRAN. The RB is a logical path provided by the first and second layers of the radio protocol to deliver data between the UE and the E-UTRAN. In general, when the RB is established, characteristics of radio protocol layers and channels required to provide a specific service are defined, and all specific parameters and operation methods are determined.
A downlink transport channel transmits data from the network to the UE. Examples of the downlink transport channel include a broadcast channel (BCH) for transmitting system information and a downlink-shared channel (DL-SCH) for transmitting user traffic or control messages. User traffic of downlink multicast or broadcast services or control messages can be transmitted on the DL-SCH or a downlink multicast channel (MCH). An uplink transport channel transmits data from the UE to the network. Examples of the uplink transport channel include a random access channel (RACH) for transmitting initial control messages and an uplink-shared channel (UL-SCH) for transmitting user traffic or control messages. A paging channel (PCH) may be provided to deliver paging information.
FIG. 4 shows an example of mapping of logical channels onto physical channels in a WCDMA system. The section 6.1 of 3GPP TS 25.211 V6.7.0 (2005-12) “Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 6)” can be incorporated herein by reference.
Referring to FIG. 4, logical channels are a dedicated channel (DCH), an enhanced dedicated channel (E-DCH), a random access channel (RACH), a broadcast channel (BCH), a forward access channel (FACH), a paging channel (PCH), and a high speed downlink shared channel (HS-DSCH). The logical channels are mapped to various physical channels.
FIG. 5 shows an example of mapping of logical channels onto physical channels in an E-UTRAN. The section 5.3.1 of 3GPP TS 3GPP V0.9.0 (2007-03) “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description: Stage 2 (Release 8)” can be incorporated herein by reference.
Referring to FIG. 5, downlink transport channels (i.e., a DL-SCH, a PCH, and an MCH) except for a BCH are mapped to a physical downlink shared channel (PDSCH). A control channel among downlink physical channels may be a physical downlink control channel (PDCCH). Comparing FIG. 4 and FIG. 5, unlike the WCDMA system using various physical channels, the E-UTRAN uses only two downlink physical channels, i.e., the PDSCH for traffic data and the PDCCH for a control signal.
In order to receive the PDSCH, a UE first has to monitor the PDCCH. After successfully decoding the PDCCH, the UE can receive the PDSCH by using scheduling information included in the PDCCH. However, since, the PDCCH is an almost unique control channel, the PDCCH is transmitted every transmission time interval (TTI). The TTI is a unit of scheduling performed by a BS The TTI is defined as a time for transmitting one sub-frame. For example, 1 TTI may be 1 ms.
Unlike the WCDMA system capable of monitoring only a control channel designed for a specific purpose, the UE in the E-URTAN needs to monitor the PDCCH every TTI in order to check the scheduling information of the UE. However, when the scheduling information of the UE is checked every TTI, the UE may experience significant battery consumption due to a relatively short TTI length.